This invention relates to safety circuits and the like which make use of multi aperture ferrite components such as those known as laddics. Laddics in common with other like devices exploit the square or, more accurately, the trapezoidal shape of their magnetisation characteristic by the application of alternate mmf pulses of opposite sign to windings embracing spaced legs of the laddic so that magnetic flux changes induced in the ferrite may be detected by an output winding in terms of an emf. Further control windings may be employed on the ferrite legs between the input and output windings so that the laddic may act as a logic gate in which a flux reversal in ferrite beneath the output winding is inhibited unless the required control winding inputs are present. Because the existence of an output signal may be made dependent upon the maintenance of separate pulsed inputs on the other windings the device is normally failsafe and finds application in safety circuits, for the output, which itself is a pulse train, will not appear if any one input or control signal fails to switch the magnetic flux in its underlying portion of the ferrite.
Should there be a failure and the ferrite which is beneath the output winding fail to be switched from a certain saturation point, the magnetic flux will fall from saturation to remnance by natural decay. Although the flux change over this part of the B-H loop is very small indeed compared to a full flux reversal, it can occur at a very much faster rate. Unless precautions are taken the amplitude of the voltage induced in the output winding as a result of this small but fast flux change can be quite comparable to a full flux change. Such a possibility cannot be tolerated in a circuit of high reliability as in a safety circuit and in these circuits laddics are coupled in cascade with the object that an output pulse from the output winding of the first laddic serves to reset the flux in the second laddic. In this context a spurious output pulse is termed reset noise for the amplitude of the noise may be sufficient for the signal to reset the next laddic. This weakness has in the past been compensated for by the use of a coupling circuit between cascaded laddics which includes a transistor having a low cut off frequency. However these transistors are becoming obsolete and the present tendency is for the trade to market planar transistors with high cut off frequencies.
An object of the present invention is to produce a means whereby the risk of spurious output signals from the output winding of a laddic is reduced and allowes a planar transistor to be employed to couple laddic devices in cascade.